This invention relates in general to thick film resistors and more particularly to such component elements which include both coarse and fine adjustment capabilities and especially adapted to laser beam trimming operations.
Thick film resistors are of course known in the art. Usually such circuit elements of this type are formed by substrate, such as ceramic, in the form of a thin sheet layed down from a suitable resistance ink or paste i.e., one that has a desired resistance characteristic per unit area. The absolute value of the resistance element so formed is customarily less than the ultimate or desired resistance so that the element can be customed trimmed to the exact value needed for the associated circuit in which it is to be utilized.
Such trimming to need is very frequently effected by laser beam. The laser cuts a longitudinal slot through the layer of resistance ink, creating a longer path that the current must travel, thereby raising the resistance value of the element. There are two important factors to be considered which essentially work at cross purposes to each other. The actual laser beam cutting must be etfective in as short a period of time as conveniently attainable so as to optimize production time. However, too fast a change in resistance value will often, and in some cases almost certainly result in an overshoot with regard to the desired resistance value. That is, the cut would be too long and resistance value too high before the laser beam can be suitably controlled (stopped).
This problem has been addressed in the past on one basis or another but which in the main have all proved to be unsatisfactory. For example, one known prior art device includes a thin film resistor element having a tapered configuration such that a laser beam cut starting at the narrowed base will provide a faster resistance change for the element as a whole than when the beam begins to cut into the wider top section, in essence slowing the resistance change on a progressively slower basis as the laser beam travels along its intended cut path. However, while this concept might be considered to be is on the "right" track, actually the change rate of resistance change is in fact not that significant.
There are still other approaches that have been tried as well. One such prior device utilizes various (multiple) cuts, some traverse, and some along the longitudinal axis of the resistance element being trimmed. Still another approach is where multiple cuts are also made, but all utilize parallel cut paths. In both instances, the cuts are made in a homogeneous layer of resistance material where the resistance value per unit area remain uniform throughout. The same considerations as referenced previously as to factors which work at cross purposes to one another still apply.
Accordingly, what is needed is flat, sheet like thick film resistance element of a substantially uniform, rectilinear shape, which nevertheless was provision for coarse and fine trim adjustments that can be effected in the resistance element in a short period of time so as to optimize production parameters and which is fully compatible with computer-controlled laser beam trimming procedures.